Drop spreading at the impact in the Leidenfrost boiling

Abstract

Although the Leidenfrost effect has been extensively studied in the past, one challenge for the modeling of this phenomenon remains, namely, how to determine the effect induced by the presence of a vapor film on the frictions exerted on the drop. To address this issue, experiments are carried out on liquids with very different viscosities including water, ethanol, and several mixtures of water and glycerol. The deformation of droplets of a few hundred micrometers, impinging a perfectly smooth solid surface heated above the Leidenfrost temperature, is observed by shadowgraphy using a high-speed camera. Experimental results are compared to a theoretical model which is based on an inviscid asymptotic solution for the flow inside the lamella. This model also considers a lamella thickness which does not depend on the viscosity, the surface tension, and thus on the Reynolds and Weber numbers. This description of the lamella is valid if Weber and Reynolds numbers are high enough. Mass and momentum balances applied to the rim bounding the spreading lamella yield an equation for the rim motion which is then solved numerically. This equation accounts for the momentum transferred to the rim by the liquid coming from the lamella, the capillary forces, and the viscous stress at the separation between the lamella and the rim. The comparison between the model and the experiments suggests that the liquid at the bottom edge of the lamella is dragged by the vapor film given that the vapor velocity in the vapor film is significantly larger than that of the liquid. This process significantly increases the drop spreading for the low viscosity liquids. An analysis of the viscous boundary layer which develops at the bottom edge of the lamella is found to confirm this scenario.

Direct numerical simulation of the impact of a droplet onto a hot surface above the Leidenfrost temperature

“…Important parameters such as the incident angle, the normal and tangential velocities of ongoing and outcoming droplets, the residence time, the spreading diameter are deduced from the image processing [9]. The temperature evolution of the droplets is measured using the two-color Planar Laser-Induced Fluorescence (2cPLIF) thermometry [15][16][17].…”

Transient evolution of the heat transfer and the vapor film thickness at the drop impact in the regime of film boiling

“…Therefore, it has been suspected that the liquid can be entrained radially by the vapor flow. In this scenario, according to Castanet et al, 11 the radial velocity at ξ = 0 can be determined by…”

“…where S e is the effective surface of heat exchange between the droplet and the solid substrate, i.e., S e = πd 2 c /4. The time evolution of d c can be evaluated using the theoretical approach developed by Castanet et al, 11 which is based on the application of momentum conservation to the rim bounding the liquid lamella. In the following, d c is evaluated from the sideview images taken by laser-induced fluorescence, which allows evaluating the length of the "apparent contact" line between the solid surface and the drop [ Fig.…”

“…10 Therefore, only two dynamical behaviors are observed: rebound (with or without partial fragmentation) and splashing. Various aspects of the Leidenfrost phenomenon have been investigated, for example, the droplet spreading over the vapor film, [11][12][13] the transition between rebound and splashing regimes, 14 the contact time of the drop near the wall, 15,16 the spreading time, 11,15,17 the restitution of velocity in the case of a rebound, 13,16 the characteristics of secondary droplets in a splashing, 14 and the dynamic Leidenfrost temperature. 18,19 When the Leidenfrost phenomenon is concerned, it should be mentioned that it does not exclusively concern drops impinging onto a hot solid surface.…”

Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

“…At higher Weber numbers (Figures 6-8), bottom views show a higher temperature band around the edge of the droplet at t = 1 ms and 2 ms. At these times, the ejected lamella is much thinner than the central region of the droplet, which helps observing a heating of the liquid in the bottom views. Provided a sufficiently large impact velocity, the lamella rapidly takes a gaussian shape surrounded by an annular rim, which is growing due to the deceleration by the surface force opposed to the spreading [10]. Liquid is progressively heated while flowing along the hot wall from the core of the lamella in the direction of the rim at the edge of the droplet.…”